The present invention relates to alkoxysilane-functionalized, allophanate-containing coating compositions, to a process for producing them, and to their use.
Polyurethanes have been established for many decades as high-value components for paint, adhesive, sealant and plastics systems. It is possible here for additional alkoxysilane groups to play an important role, for example with regard to network density, chemical resistance and scratch resistance, primarily through the formation of siloxane and polysiloxane structures.
Molecules both possessing alkoxysilane groups and having isocyanate groups offer the option of introducing the functionalities that are the resulting reaction products, siloxanes and polyurethane groups, by means of one component. Such substances have long been in use, for example in the form of isocyanatoalkyltrialkoxysilanes.
Alkoxysilane-terminated polyurethanes prepared from isocyanatoalkyltrialkoxysilanes and alcohols are likewise known and are used, for example, for producing highly crosslinked, hard coating materials (e.g. EP 2676982 A1). The alkoxysilane-containing coating materials described in EP 2676982 A1, however, are disadvantageous in having only short potlives.
Allophanate-containing binders are long-established systems. Alkoxysilane-functionalized allophanates as well are known. It is appropriate here to distinguish between a variety of types, which are set out below, but which correspond neither in structure nor in application to the alkoxysilane-functionalized allophanate present in the alkoxysilane-functionalized, allophanate-containing coating composition of the invention.
Accordingly the allophanates III (1) described in WO 2008/043722 A1 are obtained by reaction of NCO-terminated allophanate-containing polyurethanes I (1) with isocyanate-reactive alkoxysilanes II (1) (e.g. aminoalkyltrialkoxysilane). The allophanate groups here are therefore located in the centre of the polyurethane chain, and the alkoxysilane function is attached via the terminal isocyanate group, in the form of a urea function (structure III (1), equation 1).

DE 102005041953 A1 describes the reaction of a polyol I (2) having a mean molecular weight of 3000-20 000 g/mol with an excess of isocyanatopropyltrimethoxysilane II (2) so as to result, after polyurethane formation III (2), in the formation of an allophanate IV (2) having two alkoxysilane functions per allophanate unit.

In DE 102005041954 A1, a polyurethane I (3) is admixed with isocyanatopropyltrimethoxysilane II (3) and the mixture is heated until allophanate structures are formed. In this case, the alkoxysilane group is built on the terminal nitrogen of the allophanate group III (3) (equation 3).

J. Kozakiewicz et al. published, in Progress in Organic Coatings 72 (2011) 120-130, the reaction of isocyanatopropyltrimethoxysilane I (4) with methanol to form the corresponding urethane II (4) and subsequently with hexamethylene diisocyanate trimer III (4). In the resultant, highly viscous allophanate IV (4), the alkoxysilane function is pendent on the tertiary, central amine of the allophanate group (equation 4).

In the application described, the allophanate function serves as blocking agent for the hexamethylene diisocyanate trimer used as crosslinker for hydroxy-functionalized polyester polyols.
Even today there is a need for new silane-containing coating compositions which possess specific properties.
The alkoxysilane-containing binders described in EP 2676982 A1 are disadvantageous in having only short potlives.